1. Field of the Invention
A portable, flueless, low nitrogen oxides, low carbon monoxide space heater in which a swirling fuel flow mixes with a counter-swirling primary airflow, with respect to the swirling fuel flow, to form an intimately mixed fuel/air mixture. The flueless space heater admits secondary air into an upper portion of the combustion chamber for additional quenching, continuous combustion, and dilution, thus significantly reducing or eliminating nitrogen oxides and carbon monoxide emission.
2. Description of the Prior Art
The basic principles used to reduce or eliminate nitrogen oxides and carbon monoxide emissions through combustion, quenching, post burning, and dilution are known to the art. Burned with theoretical quantity of air, a methane-type gaseous fuel having a combustion temperature of approximately 3450.degree. F. and an ignition temperature of approximately 1200.degree. F. will produce dangerously high levels of nitrogen oxides in an uncontrolled burning situation. Given such conditions, an unvented space heater will produce high levels of nitrogen oxides. In comparison, kerosene vapors surrounding a wick in a similar unvented, portable space heater will ignite at approximately 700.degree. F. The low ignition temperature of kerosene vapors effectively inhibits formation of nitrogen oxides in such heaters.
In large industrial-type applications, an operating engineer has the freedom to control and adequately mix a fuel and oxidant, providing the residence or burnout time, and the staged quenching necessary to satisfy the theoretical requirements of low nitrogen oxides produced by conventional means. However, it is not immediately apparent that the low emissions of nitrogen oxides in kerosene wick-type heaters can be duplicated by a natural gas or other gaseous fuel burning appliances having a similar design, size and rating.
Reich, U.S. Pat. No. 3,689,040 teaches a low carbon monoxide, high heat output, portable gas heater of the type swept by a high velocity airstream. The gas burner may introduce fuel gas obliquely to provide a fuel swirl in a primary combustion chamber. Air is drawn into the primary combustion chamber through holes in a peripheral chamber wall. A downstream secondary combustion chamber is formed by a conical flange extending from the open end of the primary combustion chamber. A baffle plate extends over a large portion of the flange opening with secondary combustion air being admitted through holes in the flange. The holes are spaced outwardly from the baffle. If natural gas is used as a fuel, a further circle of smaller holes is provided at or near the edge of the baffle. The Reich patent teaches two-stage combustion but does not disclose a counter-swirling introduction of primary combustion air to mix with the fuel. The Reich patent does not teach any decrease in nitrogen oxides emissions.
The Reich patent teaches a space heater of the type typically used at construction sites. Such space heaters are normally rated at approximately 750,000 BTU/hr on propane and approximately 440,000 BTU/hr on natural gas. A space heater according to this invention, typically is natural gas fired at approximately 18,000 BTU/hr. The Reich patent teaches an upstream air fan and a 211/2" diameter housing rated at 5,000 cfm. Such air flow, within a heater according to the Reich patent, has a Reynolds number of approximately 360,000 which relates to highly turbulent flow.
According to the invention of this specification, the portable space heater has a Reynolds number of approximately 1,300 which relates to laminar flow; a turbulent regime begins at a Reynolds number approximately equal to 2,300. Thus a space heater according to the Reich patent could not practically be scaled down across the change in flow regimes, nor could the space heater of this invention be scaled up to burn approximately 440,000 BTU/hr as taught by the Reich patent. Reynolds numbers are often used to compare relative sizes of space heaters. The invention according to the Reich patent, and the invention of this application differ in Reynolds numbers by a factor of approximately 280. More importantly, the invention according to the Reich patent operates in a turbulent flow regime whereas the invention of this application operates in a laminar flow regime.
Hoffert et al, U.S. Pat. No. 4,671,192 discloses a pressurized cyclonic combustion method and a cylindrical burner apparatus for pressurized combustion of particulate solid fuels. The Hoffert et al patent teaches a primary combustion chamber which is designed to withstand 3,000.degree. F. A stoichiometric or above quantity of air is supplied and the expected flue gas temperature before quenching is 2700-2800.degree. F. According to the teachings of the Hoffert et al patent, a burner will generate unexceptable levels, relative to current statutory regulations, of nitrogen oxides unless a chemical scrubber is used. The burner according to the teachings of the Hoffert et al patent can not be driven by reducing primary air to sub-stoichiometric quantities and then increasing the quantity of quench air since it is not designed to handle combustion in a post-quench zone where carbon monoxide from the primary chamber would have to be burned out. Quenching according to the teachings of the Hoffert et al patent is determined by metallurgy considerations. According to Table I of the Hoffert et al patent, the Reynolds number in the combustion chamber is about 400,000 and the Reynolds number at the auxiliary air inlet is about 4,000,000. The invention of this application operates at a Reynolds number of approximately 1300 and thus is designed in a different manner.
Schol, U.S. Pat. No. 3,838,652 teaches a furnace installation for burning liquid or gaseous fuel. The furnace installation is particularly designed for boilers. The flame within the furnace is cooled by means of the flue gas mantle enveloping it without the necessity of first mixing the flue gases with combustion air.
Perret et al, U.S. Pat. No. 3,936,275 discloses a method and apparatus for combustion of liquid sulfur in a combustion chamber. The liquid sulfur enters the combustion chamber as a fine spray at the apex of a conical end wall of the combustion chamber. The fine spray is driven into contact with the conical end wall due to whirling action of the primary air. Secondary air is introduced along the lateral walls of the combustion chamber in order to protect such lateral walls from radiation heat produced during combustion.
Crawford et al, U.S. Pat. No. 4,082,497 teaches a high capacity, quiet burner for a hot air heating system used in heating the top and bottom ends of a plastic coated carton, or the like. Chalupsky et al, U.S. Pat. No. 4,340,362 teaches a portable space heater having an outlet at one end of the combustion chamber which is fitted with a conical shaped cone supported therein which traps and causes secondary combustion of a volatized admixture of fuel and air and prevents flames from shooting directly out the end of the combustion chamber.
Wright, U.S. Pat. No. 4,565,137 teaches a bio-mass suspension burner for use with furnaces or boilers. The burner includes a delivery system for injecting particulate solid fuel into a combustor. A primary air stream mixes with the fuel and conducts the mixture into the combuster where secondary air is introduced at the point of ignition. Tertiary air is introduced tangentially to maintain a cyclonic vortex. Sage, U.S. Pat. No. 2,971,480 teaches a cyclone furnace for burning ash-containing solid fuel.
Voorheis, U.S. Pat. No. 4,488,869 teaches a high efficiency, low nitrogen oxides emitting, staged combustion burner for gaseous, liquid or pulverized solid fuels. The burner is self-contained and primarily used in smaller furnaces, as opposed to large industrial furnaces, which provide low nitrogen oxides by staged combustion with introduction of swirling primary air and introduction of fuel straight along the central axis of the main combustion chamber which is in the self-contained burner. The self-contained burner also introduces secondary air at its downstream end in a direction to converge on its axis and to also provide a spin, the combination maintaining a cylindrical-shaped flame. Primary air is introduced at about 75 percent of a stoichiometric quantity and a provision is made for a short residence time in the primary combustion zone which is entirely within the burner basket.
Vache, U.S. Pat. No. 4,105,013 teaches a portable stove having a replaceable pressurized combustible gas cartridge, a burner head, and framework for supporting a cooking vessel. Vache, U.S. Pat. No. 4,192,284 also teaches a portable stove fueled by pressurized gas.
A publication by Lawrence Berkeley Laboratory, University of California, Applied Science Division, "A Comparative Study of Combustion in Unvented Space Heating Devices", by T. Lionel, R.J. Martin and N.J. Brown, presented at the Western States Section/The Combustion Institute 1984 Fall Meeting, Stanford University, Stanford, Calif., Oct. 22-23, 1984, teaches wiok-kerosene heaters and compares multi-stage, radiant, and convective configurations. The publication relates primarily to kerosene heaters and the concentrations of nitrogen oxides and carbon monoxide emissions associated with such kerosene heaters.